This invention relates to an improved arrangement for positioning a filter at the outlet of the liquid processing vessel of a sandmill.
Sandmilling is a proven, practical, continuous, high production method of dispersing and milling particles in liquids to produce smooth, uniform, finely dispersed products. One use of sandmilling is to mill agglomerates in paints. The process is also applicable to the production of a wide variety of inks, dye stuffs, paper coatings, chemicals, magnetic tape coatings, insecticides and other materials in which milling to a high degree of fineness is required.
In a typical sandmilling process, the material or slurry to be processed is introduced at the bottom of a processing chamber or vessel and pumped upward through grinding media, which in years past was usually sand, although currently it is usually a small diameter manufactured product of steel or other material rather than sand. Rotors within the vessel grind the slurry as it is pumped through the media.
Typically, the processed slurry or liquid exits the vessel at its upper end. Since the liquid may contain suspended particles of the grinding media, it is advantageous to place a filtering screen at the vessel's outlet. One such early filtering screen is an annular member attached to the upper portion of the vessel, as disclosed in U.S. Pat. No. 3,135,474 to Schold. In other mills a semi-cylindrical screen is used extending halfway around the vessel near the vessel outlet. Such screens must be removed frequently for removal of media and maintenance. A problem with the typical arrangement is that removal of the filter screen is difficult. A partial solution to this problem has been to provide a large diameter access plate or cover in the side of the vessel, adjacent the screen, so that with the plate removed a brush or other tool can be inserted to attempt to dislodge the media from the exposed portion of the screen. An adequate cleaning cannot be obtained in this manner.
One product on the market merely employs a screen across a large diameter outlet in the side of the vessel. Such screen is removable relatively easily for cleaning, but it has less filter surface than cylindrical, or semi-cylindrical, screens around the rotor. The result of this is that the screen must be cleaned often to prevent undesirably high vessel pressure or reduced flow rate.
Another solution to this problem is disclosed in U.S. Pat. No. 4,140,293 to applicant, in which a cylindrical filter screen and surrounding housing are divided axially into two separate adjoining halves held in place by clamping rings, so the housing and screen halves may be readily separated and removed for cleaning and maintenance without disrupting the remainder of the vessel. While this approach is a significant advance, a need still exists for further improvements.